1. Field of the Invention
The invention relates generally to communication systems and more particularly to optical wireless communication systems.
2. Discussion of the Prior Art
Typically, communication systems are based on radio frequency type carrier waves. The carrier waves may transmit predetermined signals, such as voice signals or data signals, by typical modulation of the carrier waves in accordance with patterns indicative of the information. At the receiving terminal, the modulated signals are demodulated, hence separated from the carrier frequency and assume a desired signal identity. The demodulated signals may, for example, become transduced into humanly discernible information, such as voice frequency signals which may be transduced through audio speakers or earphones. In the case of of data signals, the information may be transmitted to a data processing system, or the transmitted information may possibly be displayed on a viewing screen. Both amplitude and frequency modulations are well known to communicate information by radio frequency transmitters. For low cost, local communication systems, costs relating to licensing procedures as required by governmental agencies, and particularly the cost of adherence to strict standards established by the Federal Communications Commission has caused system designers to seek out alternatives to well known radio frequency transmitters. One recognized alternative mode of communication, particularly on local levels, seeks to employ light waves as carrier medium for communication systems. In the recent past, short range communications systems have been developed which are based on optical carrier waves for transmitting communications.
An advantage of optical communication systems lies of course in the avoidance of strict standards and requirements established for radio frequency communications systems, thereby allowing more freedom of design to meet cost and operational requirements. Another advantage lies in the absence of interference from radio transmissions. On the other hand, at least one disadvantage of optical communication systems has been realized in difficulties of conveniently achieving a true duplex mode of operation. For two way communication between two communicating parties, a duplex mode is a desired mode, in contrast to a non-duplex transmission mode in which communication is restricted to only a single direction at one time. Non-duplex modes of communication typically require push-to-talk switches which, when released after a communication in one direction, frees the communication channel to permit it to be used by another communicator. A duplex mode might be established with dual operating frequencies. In prior optical systems, providing two separate operating frequencies necessitated the duplication of both transmitting and receiving elements, each operating at different frequencies. With the current state of the art, the duplication of all optical components for a multi-channel system is not appropriate. So-called High-Q filters for optical systems are complex and expensive to implement.
Another problem which uniquely plagues optical communication is known in the art as leakage. Leakage is the result of interference by transmitters picked up by adjacent receivers. Since optical communications are based on light waves, unless such a system operates in total light isolation, the risk of leakage or interference from other local systems or from generally present light levels is real. In essence, leakage is noise that reduces relative signal strength of actual information transmissions. It is well known that poor transmission efficiency increases power requirements for information transmission with correspondingly decreased utility in portable systems which need to have their battery packs recharged or replaced more frequently.
The prior art has applied time division principles for transmitting from remote units to a base unit in a first time slot and for transmitting signals from the base unit to the remote units in a second time slot. The base station and each of the remote units include a first oscillator for modulating an audio or voice frequency on a first subcarrier frequency and another oscillator for modulating the subcarrier frequency on a predetermined optical infrared carrier frequency. Currently known time division systems have limitations predicated on state-of-the-art turn-on and turn-off response times of optical transmitters and receivers. The number of channels useful without interference have consequently been limited, and push to talk switches and communication feed-back to indicate the use of a channel may need to be employed to prevent undesirable leakage. In a known system, the base station includes, for example, an optical emitter for repeating the message content of optical messages received from any of the remote units to all of the remote units at the optical carrier frequency. A repetition to a number of slave units of a message received by a master unit may be undesirable in a system in which separate communications are expected to be carried on simultaneously. Hence, the referred-to system does not appear to lend itself to be used for such latter operation.